When building a Fire TV media application whose main feature is to playback video content, you as a developer have several options when it comes to building for Amazon Appstore. You can either create your app from scratch or use one of the templates that Amazon provides to get a head start. Those templates are Web App Starter Kit (WASK), Fire App Builder, and Amazon Creator. The best option for you will depend on your development background and experience.
If you are a Web developer, you can use Amazon’s Web App Starter Kit. It is a template setup to display video files. You can easily customize it by replacing sections of code in the template with your code, making the tweaks your app requires and using your own data sources. Get started with Web App Starter Kit for Amazon Fire TV here: https://developer.amazon.com/docs/fire-tv/the-web-app-starter-kit-for-fire-tv.html
If you are a Java developer, a great way to quickly build a video app is to use Amazon’s new Fire App Builder framework. You will start with a sleek looking template, where you can configure your media feed and customize the app’s appearance. A great thing about Fire App Builder is its support for components that enable you to add various functionality, such as analytics, authorization, in-app purchasing, and media playback. Start building your video app with Fire App Builder here: https://developer.amazon.com/docs/fire-app-builder/overview.html
Finally, if you are not a developer but rather you are a content creator, the recently introduced Amazon Creator allows you to build an app without writing a single line of code. All you have to do, is to provide either your YouTube channel ID or a Media RSS feed, and Amazon will generate an app for you based off that video source. The app will have a recognizable classic video list look, and support standard navigation using the Fire TV remote. Please find more information at https://creator.amazon.com